This invention relates in general to the field of communication systems, and more particularly to synchronization in a wireless system involving a base station and a plurality of mobile or fixed terminals.
In TDD (time-division-duplex) transmit and receive frames are time multiplexed. Each pair of transmit and receive frames comprises a pre-reception period, a reception period, a pre-transmission period, and a transmission period (see FIG. 2). The base station has a fixed frame structure while all terminals adjust their pre-reception and pre-transmission periods and, consequently, the receive and transmit timing for synchronization. For the synchronous FDD (frequency-division-duplex) schemes, the transmit and receive frames happen to be in different frequency bands, where each receive frame comprises a pre-reception period and a reception period and each transmit frame comprises a pre-transmit period and a transmit period.
Reception (downlink) synchronization at the terminal is generally easy to maintain. Each terminal simply adjusts its pre-reception period based on downlink synchronization signals received to determine the starting point of demodulation. In other words, the terminal determines the time from the currently received downlink synchronization signal and the terminal extends or shortens the pre-reception time to anticipate the reception of subsequent downlink synchronization signals in the subsequent frames.
Transmission (uplink) synchronization, on the other hand, requires assistance from the base station. In some applications, e.g., synchronous code-division-multiple-access (S-CDMA) communications, it is essential that signals from all terminals arrive at the base station at the same time. The uplink (from the terminal to the base station) synchronization can only be accomplished using closed-loop control, i.e., feedback signals from the base station. Thus, for transmission synchronization in prior art systems, the base station provides feedback signals to the terminal which indicate the forward timing offset. The terminal uses the received forward timing offset feedback signals to adjust its pre-transmission time accordingly.
The two principal factors that cause timing offset in a wireless system are (i) propagation delays and (ii) the clock offset between the terminal and the base station. In prior art systems, feedback signals indicating a forward offset are used to compensate for both forward propagation delays and the clock offset. As a result, prior art forward synchronization approaches often have difficulties in the presence of large environmental variations. The prime reason is the lack of open loop timing adjustment or self tuning. More specifically, it would be desirable for a terminal to actively adjust its timing by incorporating information other than the forward delay offset provided by the base station.
Therefore, an improved system and method is desired for timing compensation and synchronization in TDD wireless communications.
The present invention comprises an improved system and method for timing recovery and compensation in a synchronous wireless communication system. In the preferred embodiment, we focus on a TDD (time division duplex) S-CDMA system, where digital signals communicated between a terminal and a base station are received and transmitted in TDD frames. Each (TDD) frame comprises a pre-reception time with duration TPR, a reception time with duration TR, a pre-transmission time with duration TPT, and a transmission time with duration TT.
The two principal factors that cause the timing offset in a wireless system are (i) propagation delays and (ii) the clock offset between the terminal and the base station Due to propagation delays and the inherent clock frequency difference between the terminal and the base station, it is necessary to adjust the pre-reception and pre-transmission periods at the terminal periodically to maintain transmission and reception synchronization. The present invention provides an improved method for adjusting the pre-reception and pre-transmission times to maintain synchronization using both open loop and closed loop control techniques.
Different timing adjustment mechanisms are discussed below for three different modes of communications, namely, the monitor mode, the access mode, and the communication mode.
During the monitor mode, a terminal only receives the downlink signals broadcast from the base station without transmitting uplink signals. Briefly, in accordance with the present invention, the terminal utilizes received downlink synchronization signals to calculate the clock offset between the base station and the terminal, and actively adjusts the pre-reception time to accomplish frame synchronization. This can be done by applying the an accumulation of downlink delta signals to a local digital clock-locked-loop (CLL) to fine tune the default value of a counter that determine the duration of the pre-reception time, and hence gradually eliminates the timing difference due to clock offsets between the base station and the terminal.
During the access mode, a terminal transmits a signal to the base station, the base station receives the signal and sends a feedback signal comprising an access delta signal xcex4tf. The terminal then adjusts its pre-transmission time and pre-reception time accordingly. This delay represented by the access delta signal is caused mainly by the propagation delay or distance between the terminal and the base station.
After the access mode the terminal enters the communication mode in which the terminal and the base station exchange message information regularly. In every TDD frame, the terminal receives downlink signals from the base station during the pre-reception and the reception times. From the received downlink signals, the terminal first determines a downlink delta signal value, xcex4tb. The terminal compensates by adjusting the pre-reception period, using the downlink delta signal value. According to the present invention, the terminal adjusts the pre-transmission time using the complement of the downlink delta signal value so that the transmission timing remains unchanged.
The signals transmitted by the base station to the terminal include feedback signals to the terminal to adjust this timing of the uplink transmission. The receiver demodulates downlink signals and extracts the uplink delta signals, xcex4tf, therefrom. The terminal responds to the command and adjusts its pre-transmit timing. The uplink signals are also applied to a local digital clock-locked-loop (CLL) to fine tune the system clock of the terminal during the communication mode.
Therefore, in contrast to prior art approaches which achieve transmission synchronization based merely on feedback signals from the base station, the present invention incorporates knowledge of the propagation delays obtained during the access mode to significantly enhance the efficiency and reliability of timing control. During the communication mode, the present invention uses closed loop timing synchronization to compensate substantially only for the remaining timing offset due to clock differences. This is in contrast to prior art Systems which use closed loop timing synchronization, i.e., feedback from the base station, to compensate both for forward propagation delays and clock differences. Since the closed loop timing synchronization is not required to compensate for both the forward propagation delay and the clock difference, but rather only for the clock difference, the present invention provides improved timing synchronization.
Thus, the present invention uses both open loop and closed-loop timing control for more efficient and reliable synchronization. Open loop timing adjustments are used to compensate for propagation delays, thus enabling closed loop timing adjustments to be reserved substantially exclusively for clock offset differences. The prior art does not teach or suggest a method that combines open loop and closed loop synchronization mechanisms to cope with timing offset duo to propagation delays and clock offset. The present invention thus has the advantage of improved robustness against hardware imperfection and drastic environmental variations. The self-adaptive timing adjustment mechanism allows the use of lower cost, low precision oscillators at the terminals, leading to a considerably reduction in system cost. The present invention thus has the advantage of an improved high efficiency synchronization scheme in a TDD system.